Firewall Stateful Inspection of ICMP

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1 The feature categorizes Internet Control Management Protocol Version 4 (ICMPv4) messages as either malicious or benign. The firewall uses stateful inspection to trust benign ICMPv4 messages that are generated within a private network and permits the entry of associated ICMP replies into the network. The feature helps network administrators to debug network issues by using ICMP so that intruders cannot enter the network. This module provides an overview of the firewall stateful inspection of ICMPv4 messages and describes how to configure the firewall to inspect ICMPv4 messages. Prerequisites for, page 1 Restrictions for, page 1 Information About, page 2 How to Configure, page 3 Configuration Examples for, page 8 Additional References for, page 9 Feature Information for, page 9 Prerequisites for You must configure the Cisco firewall before you can configure the feature. The network must allow all ICMP traffic to pass through security appliance interfaces. Access rules must be configured for ICMP traffic that terminates at a security appliance interface. Restrictions for This feature does not work with the UDP traceroute utility, in which UDP datagrams are sent instead of ICMP packets. UDP traceroute is the default for UNIX systems. For a UNIX host to generate ICMP traceroute packets that are inspected by the firewall, use the -I option with the traceroute command. 1

2 Information About Information About Overview of the Internet Control Management Protocol (ICMP) is a network protocol that provides information about a network and reports errors in the network. Network administrators use ICMP to debug network connectivity issues. To guard against potential intruders using ICMP to discover the topology of a private network, ICMPv4 messages can be blocked from entering a private network; however, network administrators may then be unable to debug the network. You can configure Cisco routers to use access control lists (ACLs) to either completely allow or deny ICMPv4 messages. When using ACLs for ICMPv4 messages, message inspection has precedence over the configured allow or deny actions. ICMPv4 messages that use the IP protocol can be categorized into the following two types: Informational messages that utilize a simple request/reply mechanism. Error messages that indicate that some sort of error has occurred while delivering an IP packet. Note To prevent ICMP attacks from using the Destination Unreachable error message, only one Destination Unreachable message is allowed per session by the firewall. A host that is processing a UDP session that is traversing the firewall may generate an ICMP error packet with a Destination Unreachable message. In such cases, only one Destination Unreachable message is allowed through the firewall for that session. The following ICMPv4 packet types are supported: Table 1: ICMPv4 Packet Types Packet Type Name Echo Reply Unreachable Echo Request Time Exceeded Timestamp Request Timestamp Reply Description Reply to an echo request (type 8). Possible reply to any request. Ping or a traceroute request. Reply if the time-to-live (TTL) size of a packet is zero. Request. Reply to a timestamp request (type 13). ICMPv4 packet types 0 and 8 are used to ping a destination; the source sends out an Echo Request packet and the destination responds with an Echo Reply packet. Packet types 0, 8, and 11 are used for ICMPv4 traceroute 2

3 ICMP Inspection Checking (that is, Echo Request packets that are sent start with a TTL size of 1) and the TTL size is incremented for each hop. Intermediate hops respond to the Echo Request packet with a Time Exceeded packet and the final destination responds with an Echo Reply packet. If an ICMPv4 error packet is an embedded packet, the embedded packet is processed according to the protocol and the policy configured for the packet. For example, if the embedded packet is a TCP packet, and a drop action is configured for the packet, the packet is dropped even if ICMPv4 has configured a pass action. The following scenario describes how ICMPv4 packets pass through the firewall: 1 An ICMPv4 packet arrives at the source interface. The firewall uses the source and destination addresses of the packet without any change for packet inspection. The firewall uses IP addresses (source and destination), the ICMP type, and the protocol for session key creation and lookup. 2 The packet passes the firewall inspection. 3 Return traffic comes from the destination interface and, based on the ICMPv4 message type, the firewall creates the session lookup key. 4 1 If the reply message is an informational message, the firewall uses the source and destination addresses from the packet without any change for packet inspection. Here, the destination port is the ICMPv4 message request type. 2 If the reply message is an ICMPv4 error message, the firewall uses the payload packet present in the ICMP error packet to create the session key for session lookup. 5 If the firewall session lookup is successful, the packet passes the firewall inspection. ICMP Inspection Checking ICMP return packets are checked by the inspect code, and not by access control lists (ACLs). The inspect code tracks destination address from each outgoing packet and checks each return packet. For Echo Reply and Timestamp Reply packets, the return address is checked. For Unreachable and Time Exceeded packets, the intended destination address is extracted from the packet data and checked. How to Configure Configuring Perform this task to configure the firewall stateful inspection of ICMP, which includes the following: A class map that matches the ICMP traffic. A policy map with the inspect action. Security zones and zone pairs (to attach a firewall policy map to the zone pair). 3

4 Configuring SUMMARY STEPS 1. enable 2. configure terminal 3. access-list access-list-number {deny permit} icmp source source-wildcard destination destination-wildcard 4. class-map type inspect class-map-name 5. match protocol protocol-name 6. exit 7. policy-map type inspect policy-map-name 8. class class-map-name 9. inspect 10. exit 11. exit 12. zone security zone-name 13. exit 14. zone-pair security zone-pair-name source source-zone destination destination-zone 15. service-policy type inspect policy-map-name 16. end DETAILED STEPS Step 1 Step 2 Command or Action enable Device> enable configure terminal Purpose Enables privileged EXEC mode. Enter your password if prompted. Enters global configuration mode. Step 3 Device# configure terminal access-list access-list-number {deny permit} icmp source source-wildcard destination destination-wildcard Defines an extended IP access list. Device(config)# access-list 102 permit icmp Step 4 class-map type inspect class-map-name Device(config)# class-map type inspect c1 Defines the class on which an action is to be performed and enters QoS class-map configuration mode. 4

5 Configuring Step 5 Step 6 Step 7 Step 8 Step 9 Command or Action match protocol protocol-name Device(config-cmap)# match protocol icmp exit Device(config-cmap)# exit policy-map type inspect policy-map-name Device(config)# policy-map type inspect p1 class class-map-name Device(config-pmap)# class c1 inspect Purpose Configures a match criterion for a class map on the basis of the specified protocol. Exits QoS class-map configuration mode and enters global configuration mode. Creates a protocol-specific inspect type policy map and enters QoS policy-map configuration mode. Defines the class on which an action is to be performed and enters QoS policy-map class configuration mode. Enables stateful packet inspection. Step 10 Step 11 Step 12 Step 13 Device(config-pmap-c)# inspect exit Device(config-pmap-c)# exit exit Device(config-pmap)# exit zone security zone-name Device(config)# zone security z1 exit Device(config-sec-zone)# exit Exits QoS policy-map class configuration mode and enters QoS policy-map configuration mode. Exits QoS policy-map configuration mode and enters global configuration mode. Creates a security zone and enters security zone configuration mode. Your configuration must have two security zones to create a zone pair: a source zone and a destination zone. In a zone pair, you can use the default zone as either the source or the destination zone. Exits security zone configuration mode and enters global configuration mode. 5

6 Verifying Step 14 Command or Action zone-pair security zone-pair-name source source-zone destination destination-zone Purpose Creates a zone pair to which interfaces can be assigned and enters security zone-pair configuration mode. Step 15 Device(config)# zone-pair security inout source z1 destination z2 service-policy type inspect policy-map-name Attaches a firewall policy map to a zone pair. Step 16 Device(config-sec-zone-pair)# service-policy type inspect p1 end Device(config-sec-zone-pair)# end Exits security zone-pair configuration mode and enters privileged EXEC mode. Verifying You can use the following show commands in any order. SUMMARY STEPS 1. enable 2. show ip access-lists 3. show policy-map type inspect policy-map-name 4. show policy-map type inspect zone-pair zone-pair-name 5. show zone security zone-name 6. show zone-pair security [source source-zone destination destination-zone] DETAILED STEPS Step 1 enable Device> enable Enables privileged EXEC mode. Enter your password if prompted. Step 2 show ip access-lists 6

7 Verifying Device# show ip access-lists Displays information about the specified policy map. Step 3 show policy-map type inspect policy-map-name Device# show policy-map type inspect p1 Displays information about the specified policy map. Step 4 show policy-map type inspect zone-pair zone-pair-name Device# show policy-map type inspect zone-pair inout Displays the runtime inspect type policy-map statistics for the zone pair. Step 5 show zone security zone-name Device# show zone security z1 Displays zone security information. Step 6 show zone-pair security [source source-zone destination destination-zone] Device# show zone-pair security source z1 destination z2 Displays source and destination zones and the policy attached to the zone pair. The following sample output from the show ip access-lists command shows how ACLs are created for an ICMP session for which only ping packets were issued from the host: Device# show ip access-lists Extended IP access list 102 permit icmp any host time-exceeded permit icmp any host unreachable permit icmp any host timestamp-reply permit icmp any host echo-reply (4 matches) The following is sample output from the show policy-map type inspect p1 command: Device# show policy-map type inspect p1 Policy Map type inspect p1 Class c1 Inspect The following is sample output from the show policy-map type inspect zone-pair inout command: Device# show policy-map type inspect zone-pair inout 7

8 Configuration Examples for Zone-pair: inout Service-policy : p1 Class-map: c1 (match-all) Match: protocol icmp Inspect Session creations since subsystem startup or last reset 0 Current session counts (estab/half-open/terminating) [0:0:0] Maxever session counts (estab/half-open/terminating) [0:0:0] Last session created never Last statistic reset never Last session creation rate 0 half-open session total 0 Class-map: class-default (match-any) Match: any Drop 0 packets, 0 bytes The following is sample output from the show zone security command: Device# show zone security zone self Description: System defined zone The following is sample output from the show zone-pair security command: Device# show zone-pair security source z1 destination z2 zone-pair name inout Source-Zone z1 Destination-Zone z2 service-policy p1 Configuration Examples for Configuring Device# configure terminal Device(config)# access-list 102 permit icmp Device(config)# class-map type inspect c1 Device(config-cmap)# match protocol icmp Device(config-cmap)# exit Device(config)# policy-map type inspect p1 Device(config-pmap)# class c1 Device(config-pmap-c)# inspect Device(config-pmap-c)# exit Device(config-pmap)# exit Device(config)# zone security z1 Device(config-sec-zone)# exit Device(config)# zone security z2 Device(config-sec-zone)# exit Device(config)# zone-pair security inout source z1 destination z2 Device(config-sec-zone-pair)# service-policy type inspect p1 Device(config-sec-zone-pair)# end 8

9 Additional References for Additional References for Related Documents Related Topic Cisco IOS commands Security commands Document Title Master Command List, All Releases Security Command Reference: Commands A to C Security Command Reference: Commands D to L Security Command Reference: Commands M to R Security Command Reference: Commands S to Z Standards & RFCs Standard/RFCs RFC 792 RFC 950 RFC 1700 Title Internet Control Message Protocol Internet Standard Subnetting Procedure Assigned Numbers Technical Assistance Description The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. Link Feature Information for The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature. 9

10 Feature Information for Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to. An account on Cisco.com is not required. Table 2: Feature Information for Feature Name Firewall Stateful Inspection of ICMP Releases Cisco IOS XE Release 2.1 Cisco IOS XE Release 3.2S Feature Information The feature categorizes ICMPv4 messages as either malicious or benign. The firewall uses stateful inspection to trust benign ICMP messages that are generated within a private network and permits the entry of associated ICMP replies. 10

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